To access the Observation Preparation Tool (OPT), login directly to the OPT Suite URL: https://obs.vla.nrao.edu/opt.  Note, to exit the tool properly, use the Exit link in the upper right corner or log out with FILE → EXIT; do not close the browser window/tab.

By the time you have reached this section of the OPT manual, you should have a populated source catalog (either personal or project related generated for you by NRAO staff) and a resource catalog populated with a custom resource create by you (or a colleague) and/or perhaps a copy of an NRAO default resource. In order to create a scheduling block (SB), you must first know the sources (science target(s) and calibrators) and resource(s) you wish to use in your observation in order to define the scans within the SB. It is useful to outline the project in terms of the program block (PB) and SBs in advance. Use information from the proposal, etc. (refer to General Guidelines) and create (re)source catalogs with only the subset of (re)sources that will be used in the SB you are about to create. Having your (re)sources in small personal catalogs is convenient and faster than having large catalogs or switching back and forth between your personal catalog(s) and, e.g., the VLA catalog. Check the remaining (re)sources for correctness before you continue.

The purpose of the OPT is to combine a source from one of your source catalogs with a resource from one of your resource catalogs, and to specify an observing mode, a time interval, and an intent for this combination. Repetitive combinations will build an observing schedule that defines an SB which may be executed by VLA operations. The sequence of scans in an SB will appear in the left-hand side column in the project tree and in the Schedule Summary on the Reports tab.

The most important rule when working in the OPT: be patient.  When you click on something or enter information into a field it may take a few seconds to connect back and forth between your browser and the NRAO database.  Please avoid clicking or entering before the previous action was completed. Be patient and watch the busy icon of your browser to stop (and sometimes even a bit longer).

If you have any questions, you may either submit your questions through the NRAO Science Helpdesk, request help directly from the OPT by selecting HELP → CONTACT SUPPORT, or if you have already created a scheduling block (SB) you may request help from the Validation and Submission tab. Note, if you request help from the Validation and Submission tab of an SB the project code and SB ID will be included in the subject line.

Left-hand Column

On the left-hand column of the OPT, you will see an icon menu at the top. If this is your first time using the OPT there will be nothing below the icon menu or you may see an accepted project (Figure 4.1). 

Figure 4.1:  Left-hand column of OPT.

The OPT contains a collection of projects. Projects are subdivided in a tree of program blocks (PBs), each subdivided in scheduling blocks (SBs), which each contain scans and/or loops of scans. This column, per SB, contains the scan list, i.e., the column represents the observing schedule. In contrast to the SCT and RCT, much of the editing in the OPT will be performed in the left-hand side column as well as in the main editing window.

Icon Menu

The icon menu in the OPT has more options than in the SCT and RCT. The icons in-common have the same functionality as the icons in the SCT and RCT, but as more editing is done in the left-hand side column some extra icons (with their fly-over help tool-tip) are added. Only icons valid for the selected or highlighted items (e.g., PB or scan) are displayed. A full list of icons and their meanings is shown in the listing below. These icons do not apply to the main editing window, only to the Project tree items.

	Click to allow multiple selections for cut/copy/paste
	Add a new (blank) scan
	Add a new scan (with copy of source and hardware only)
	Cut (delete) selected tree item
	Copy selected tree item
	Paste selected tree item
	Icon menu separator (no action)
	Move selected item up by one in current tree level sequence
	Move selected item down by one in current tree level sequence
	Promote selected item up, above the current tree branch (out of loop)
	Demote selected item down, in the next current tree branch (into loop)
	Icon menu separator (no action)
	Collapse/hide all items in the selected tree
	Expand/show all items in the selected tree

SB Validation Icons

New icons have been added to the OPT at the SB level indicating when an SB, scan, loop, or subarray is invalid or not validated.  When the OPT validates an SB or a scan, this does not necessarily indicate that the SB is valid for observing.  The VLA data analysts will still perform their validation check of all submitted SBs before approval. 

	This overlay indicates the SB, scan, loop, or subarray contains invalid information.
	This overlay indicates the SB, scan, loop, or subarray has not been through the internal validation process of the OPT.
	This overlay indicates the SB, scan, loop, or subarray has a questionable setting that the internal validation is reporting.

Note that based on the various themes the icons will appear a little different than below, which is using the Standard Light theme.

SB Icons		Scan Icons		Loop Icons		Subarray Icons
	valid		valid		valid		valid
	error		error		error		error
	not validated		not validated		not validated		not validated
	warning		warning		warning		warning

Menu Strip

The OPT has many menu strip options. Some of the options are only accessible when editing certain items, i.e., PBs, SBs, and scans.

To utilize the OPT for creating scheduling blocks (SBs), you may either use an existing approved science project under your OPT login or create a test project. To create a test project, go to FILE → CREATE NEW → TEST PROJECT. A test project will automatically generate a program block (PB) and an empty SB.  For the test project, you may edit the name of the project and the PB to something more descriptive to you. As with a science project, you may add co-I's (they must have a my.nrao.edu profile) to your test project if you wish to share your observing ideas with a student, teacher, advisor, or colleague.

Project Level

Once you have created or decided on which project to work on, click on the project name in the left-hand side column. You will then be presented with the Project Details.

Listed below the Project Details will be the principle investigator (PI), proposal contact author, and a list of co-authors.  All are listed with name and default email as indicated in their my.nrao.edu user profile.  The options to receive emails (yes or no) is to allow each person on the proposal to receive email (un)submissions of SBs and the observing log for each successful observation. Note, the VLA operators do not email observing logs for failed observations. If you have a question regarding a failed observation, please contact the NRAO Science Helpdesk.

Program Block Level

To reveal the PB(s), click on the plus icon to the left of the project in the project tree. All projects will have at least one PB, some may have more depending on allocated array configurations and observing priority per observing semester and/or array configuration. 

Below the Program Block Details and Acceptable Configurations is the ability to link SBs (described under Linking SBs), table of generated SB(s), and a table of execution blocks (EBs).

If you have created a test project, you may edit two of the fields: Name and Acceptable Configurations. When creating SBs under a test project, you must select a configuration. Do so by selecting the configuration (hold onto it with the mouse button) and then drag the desired configuration from the right column to the left column.

Scheduling Blocks Table

Each SB will be listed in the Scheduling Block table with 10 sort-able columns of information pertaining to the SB.

• Index (to be used when linking SBs)
• Name (when selected, will open the SB)
• Id# (each SB is given a unique identification number)
• Status (NOT_SUBMITTED, SUBMITTED, SCHEDULABLE, COMPLETED, ON HOLD, or EXPIRED)
• Executions (number of counts or repetitions for the SB)
• Total Time (total duration of all executions of an SB with N counts or N+1 counts)
• Per Execution (total duration of a single execution of the SB)
• Scheduled Start (Dynamic SBs will indicate LST start range and Fixed-date SBs will indicate LST day and time)
• Wind (selected wind (in m/s) constraints)
• Api (selected atmospheric phase interferometer (in degrees) constraints)

Execution Blocks Table

An EB is generated once an SB has been selected by the Observation Scheduling Tool (OST). If an observation is completed or failed, it will appear in the Execution Blocks table. In addition, each SB has an Executions tab. Successful and unsuccessful (i.e., failed) observations of the selected SB will also appear in the Executions tab. The Execution Blocks table contains 10 columns:

• SB ID (scheduling block identification)
• EB ID (execution block identification)
• Status (Completed or Failed)
• Duration (total length of SB)
• Started (two sets of days and times in LST and UT)
• Finished (two sets of days and times in LST and UT)
• Initial API (°) (initial atmospheric interferometry conditions)
• Initial Wind (m/s) (initial wind conditions)
• Script + VCI (If the SB was observed using the OST, the script and VCI (virtual correlator interface) files are available for download.)

To create an SB, first navigate to the Project and the appropriate PB. For approved science projects, it's important to select the appropriate PB. If there is more than one PB, be sure the SB(s) you will be creating contain the allocated science target(s) and resource(s) approved for the selected PB.  

Create a new SB:  FILE → CREATE NEW → SCHEDULING BLOCK

SB Components

After creating a new SB or selecting an existing SB you wish to edit, you are presented with six tabs in the main editing window (Figure 4.4). We will begin by first explaining the Information tab.

Figure 4.4: New SB containing no scans. (Click to enlarge)

Information Tab

The Information tab is used to set the scheduling constraints of the observation.

The other tabs seen in Figure 4.4 (Reports, Validation and Submission, and Bulk Scan Edit), will be explained in later sections. The next step is to create a scan in the SB and understand its purpose and components.

Scan Details

To create a new scan within the SB you wish to edit, go to FILE → CREATE NEW → SCAN → IN.

The idea is to define a sequence of scans in the left-hand column, each with a source, a resource, an observing mode, a time interval, and some reason (intent). Each time a scan is added you will need to specify these items. However, it is not always straightforward to assemble the scan list in the sequence you want the first time around, and you will need to move scans around. This is easily done! That is, there is no need to panic if you make scans (a bit) out of order; it is almost straightforward to add, e.g., an extra bandpass calibration scan, to move some scans to the middle of the observation, or to redefine source loops after the main framework of your schedule is set up.

Figure 4.5 shows an example of a new scan using the default scan mode, Standard Observing. The "?" next to each of the parameters provide a flyover of information. The table below describes each of the Standard Observing parameters.

Figure 4.5: New Standard Observing (STD) scan showing all available parameters and intents.

Adding Scans

There are two methods for adding scans, either create a new scan or copy and paste an existing scan anywhere in the list of scans.

Add a new scan:

• Select the scan you wish to add a new scan before or after.
• FILE → CREATE NEW → SCAN → BEFORE
• This will add a new scan before the scan you selected.
• FILE → CREATE NEW → SCAN → AFTER
• This will add a new scan after the scan you selected.
• FILE → CREATE NEW → SCAN → IN
• This will add a new scan in the selected SB or a selected loop. (Scan loops will be explained in the next section.)

Copy/paste a scan:

• Select the scan you wish to copy.
• EDIT → COPY <name of scan>
• EDIT → PASTE BEFORE <name of scan>
• This will paste the scan before the scan you have selected or before the loop you have selected. (Scan loops will be explained in the next section.)
• EDIT → PASTE INTO <name of scan>
• This will paste the scan into a selected loop.
• EDIT → PASTE AFTER <name of scan>
• This will paste the scan after the scan you have selected or after the loop you have selected.

You may also use the copy and paste icons located in the icon menu. The paste icon option will only paste the scan after the scan you have selected.

Scan Loops

To create a scan loop, select the scan you wish to either create the loop before or after and select,

FILE → CREATE NEW → SCAN LOOP → BEFORE  (or AFTER)

Once a loop has been created, you can create new scans within the loop, copy/paste scans into the loop, and create more loops within the loop. 

Scan loops are useful for creating a repetition of scans, i.e., for phase referencing (complex gain calibrator scan plus science target scan(s)). Loops can be named, be given many iterations, and contain a useful feature called Bracketed. If the Bracketed option is selected it will perform your first scan within the loop one extra time at the end of the loop.

Figure 4.6 shows four examples of bracketing the science target with the complex gain calibrator.

No Loop	Normal Loop		Bracketed Loop
individual Scans	two different orderings		should start with calibrator
Figure 4.6: Four different bracketing styles. In these examples, Cal is an abbreviation for complex gain calibrator.

Bracketing Options

• No Loop: This option does not utilize scan loops, however it is a valid option for bracketing the science target with the calibrator.
• Normal Loop: Two options are represented here.
• On the left, the calibrator is placed before the loop containing the science target and the calibrator.
• On the right, the calibrator is placed after the loop containing the science target and calibrator.
• Both options, the science target is appropriately bracketed by the calibrator.
• Bracketed Loop: The scan loop utilizing the bracket option is signified by two asterisks (**) next to the number of iterations the loop was assigned. With the bracket option selected, the first scan in the loop must be the calibrator scan so it will also be the last scan in the loop. This may not be apparent when only looking at the SB list of scans in the left-hand column, but will become more obvious when viewing the SB summary in the Reports tab.

A loop can contain any number of sources, not necessarily only a calibrator scan and a single science target scan. If your science target sources are near in the sky and you can use a single calibrator for all of the science targets, then you can group them in a loop with more than one, target scans before returning to your calibrator. Keep in mind that the total loop time should be shorter than the anticipated coherence time at our observing frequency. Loops may also contain loops. If your loop is selected, adding a new scan will place this new scan in the scan loop. The only difference with a normal scan is that this scan will be scheduled as many times as the "Loop iterations" specified, consecutively in a loop with the other sources in the loop. When finished with defining a loop, you may want to highlight it and then collapse it (using from the icon menu) for a more compact display in the tree.

Calibration

In addition to bracketing the science target(s) with its appropriate complex gain calibrator, it's important to understand when and where to add interferometric pointing (reference pointing) scans for high frequency (> 15 GHz) observations, where to place the flux density scale calibrator or bandpass calibrator (beginning, middle, or end of the SB).  The Observing Guide has specific sections to help guide you on how to setup a high frequency or low frequency observation. However, the placement of some calibrators, i.e., flux density scale calibrator, will be determined by where that source is in the sky relative to the science target(s). Is it rising or setting? Double check the EL curve of the sources in the SCT (under the Images tab of a source) to help determine a good placement of the flux density scale calibrator. The placement of the flux density scale calibrator will also restrict the LST start range. There are many important factors to consider when creating an SB.

Below is a listing of specific sections within the Observing Guide helpful for setting up observations:

Calibration

High Frequency Strategy

Low Frequency Strategy

Very Low Frequency Strategy

The Observing Guide also contains sections on Spectral Line, Polarimetry, Subarrays, Mosaicking and OTF, Moving Objects, VLBI at the VLA, and Pulsars under the Commonly Used Observing Modes section. 

If at any time you are unsure of what to do or where to start with creating your SBs, please do not hesitate to contact us through the NRAO Science Helpdesk.

OPT 1.31 User Interface Update

Filter Source Search

New with 1.31 is the ability for the user to filter their source search in a source catalog. This can be useful if the source catalog in question has many source entries, such as sources used for a survey. Figure 4.7.A shows how to activate the Filter search. Figure 4.7.B shows the parameters available for the Filter. From the top in Figure 4.7.B:

• Source Catalog: the catalog upon which you wish to search
• Source Group: if you have groups in your catalog, you can search them individually or all of them
• Sources - Stop Filter: this will collapse the Filter options
• Name: filter by name of source
• RA: filter by RA position - Reset button resets RA to 0h 00m 0.00s (Note: when entering values you must use units)
• Dec: filter by Dec position - Reset button resets Dec to 0d 00' 0.00" (Note: when entering values you must use units)
• Radius (deg): search radius in degrees based upon RA and/or Dec position - Reset button resets Radius to 1.0d

The search results will appear as a list before the parameters section. Click on the button next to the source and select Change, or Cancel to quit.

This same filter functionality is also available in the Bulk Scan Edit tab.

Figure 4.7.A: Activating the Search Filter	Figure 4.7.B: Search filter options

Filter Resource Search

New with 1.31 is the ability for the user to filter their resource search in a resource catalog. This can be useful if the resource catalog in question has many resources. Figure 4.8.A shows how to activate the Filter search. Figure 4.8.B shows the parameters available for the Filter. Figure 4.8.C shows what happens when you select a sampler that is not available for the receiver. Figure 4.8.D shows that you can have multiple bands and/or samplers selected. From the top in the parameters Figure 4.8.B:

• Resource Catalog: the catalog upon which you wish to search
• Resource Group: if you have groups in your catalog, you can search them individually or all of them
• Resources - Stop Filter: this will collapse the Filter options
• Name: filter by name of source
• Filter Receiver Bands: filter by receiver band(s)
• Filter Sampler: filter by 3-bit and/or 8-bit samplers

The search results will appear as a list before the parameters section. Click on the button next to the source and select Change, or Cancel to quit.

This same filter functionality is also available in the Bulk Scan Edit tab.

Figure 4.8.A: Activating the Search Filter	Figure 4.8.B: Search filter options

Figure 4.8.C: Incorrect sampler for receiver band	Figure 4.8.D: Example of multiband / sampler search results

While creating an SB, it is important to keep in mind the allocated time in the PB you are working in. While we do allow observers to submit over the allocated time per PB, i.e., multiple SBs for multiple sources or multiple SBs to cover a wide range of LST start ranges to help improve chances of observing. The sum of the SBs or a single SB must not exceed the allocated time of a PB. For example, if a PB is allocated 4 hours, the observer may submit a 4 hour SB, a 2 hour SB with a count of 2, 2x 2 hour SBs, many SBs of varying lengths, many linked SBs (i.e., only observe one of the 2 or 3 SBs, etc.), or any combination of those options. Just be sure the sum of SBs you wish to be observed do not exceed the allocated time. For details on how to link SBs, refer to the Linking SBs section of this manual.

Static Report Tab

The Static Report tab (Figure 4.9) should be accessed regularly to double check the layout of the SB. This tab provides a table view of all (re)sources imported into the scans, including an instrument configuration summary, time on source summary, and a summary report of the SB. At the bottom of this tab, in the interface feedback strip, warnings and errors will appear if there are issues with the SB, i.e., antenna shadowing (if in D or C-configuration), EL limit warnings (if the EL is > 80d or < 8d (unless otherwise manually set)), if the LST start range overlaps with sunrise or sunset (if they have been selected), etc.. Below Figure 4.9 is a table explaining the top portion of the Report tab.

Instrument Configuration Summary

All resources imported into scans within an SB will be displayed in the Instrument Configuration Summary. This table can reveal if the correct or incorrect resources have been imported. Figure 4.10 is an example of the resources used in the demo SB seen in Figure 4.9. To reveal the subbands within the basebands, click on Show All Subbands. To minimize the view, click on Hide All Subbands or hide individual basebands. If a resource is set up to utilize the Doppler settings, it is advised to double check the OPT's calculation of the line's center frequency using Dopset. Refer to the Instrument Validation section of the Observing Guide for more details on how to verify a spectral line setup.

Figure 4.10: Example of an Instrument Configuration Summary table (click image to enlarge).

Time On Source Summary

The Time On Source Summary lists all sources used in the scans with their assigned resource, scan name, modifiers (i.e., scan intent(s)), coordinates in RA and Dec, Doppler Velocity used in the assigned resource, Doppler Line frequency of the assigned resource, min and max hour angle (HA), min and max parallactic angle (PA), and total time on source. This table is helpful in determining how much time on source a science target or calibrator with a given resource has for a specific LST start (set in the assumed LST start field in the top portion of the Static Report tab). Note, as the assumed LST start time is changed, the time on source for each source will change. This is due to the change in the antenna slew time between sources due to the change in the sources positions relative to each other on the sky.

Figure 4.11: Example of a Time On Source Summary table.

Summary Report

The Summary Report (Figure 4.12) is the most valuable for checking the overall setup of an SB. This is a table view of the scan listing created in the left-hand column. If scan loops have been utilized, be sure to click on the plus icon at the top left of the Summary Report table to expand all loops (including loops within loops). It is important to step through the LST start range (defined by the observer in the Information tab) by entering different assumed LST start times at the top of the Static Report tab. This will help determine whether each observing scan (not including setup scan(s)) have the appropriate time on source (after slewing) for the entire LST start range assigned in the Information tab. The AZ and EL should also be carefully checked to avoid antenna wrap issues and EL limits. If an observing scan does not have enough time on source (after slewing), that scan will be highlighted in red. It is ok if setup scans do not have time on source (they will not appear in red). The setup scans (for setting the attenuator and requantizer gain levels) do not require the antennas to be on source.

OPT 1.31 User Interface Update

New for 1.31 is the addition of the scan mode, e.g., standard observation (STD), interferometric pointing (IP), to the scan number in the first column of the Summary Report table. This matches the scan mode listing found in the tree structure in the left hand column of the OPT which has scan mode labels, e.g., STD, IP, etc. (Figure 4.12)

Figure 4.12: Example of a Summary Report. (Click to enlarge.)

We have added a new tab to the scheduling block (SB), "Dynamic LST Validation" (aka Block Checker Tool). This new built-in tool will allow observers to check their observations for a specified LST start range and several defined parameters.

Important Notes:

This new tool is a work in progress. Please contact the NRAO Science Helpdesk if you encounter any issues, discrepancies, or you see a message with "unknown" severity.

The NRAO staff will continue to check SBs before approving for observation.

Currently, the tool will not check or flag the following parameters:

• antenna shadowing (applicable to D and C-configuration)
• However, antenna shadowing information remains present on the Static Reports tab in the Summary Report table near the bottom. You will need to adjust the assumed LST start time to check when/if there are shadowed antennas.
• antenna wraps (CW, CCW, or no preference)
• We are working on optimizing this for less confusion. For now, ask us for help in determining which antenna wrap to use if you are unsure.

Dynamic LST Validation Parameters

Starting Conditions

The following are editable starting conditions.

Evaluator Settings

The following are editable evaluator settings.

If you wish to reset the evaluators to the default settings, select "Reset to VLA Defaults". Note, the starting conditions will not be reset.

Running the Block Checker

Once you are satisfied with your starting conditions and evaluator settings, select "Start Dynamic LST Validation".

The results may be grouped by evaluator, severity, or all. A "Show All" or "Hide All" results option is also available.

This can be helpful when there are many messages and only want to focus on a particular evaluator or severity. The messages in the table(s) may be sorted by selecting the top row of each column, i.e., by scan name, scan number, etc..

In addition to the editable evaluator parameters, you may see addition evaluator messages if they have been flagged in your SB. If they do not appear, then they were not used. This is likely ok. It is also ok if after you select "Start Dynamic LST Validation", you see the info message: "No Messages to display. Scheduling Block looks good."

Message Types

Evaluators

Validate and Submit

If you are satisfied with the SB you have created, triple checked everything under the Static Report tab, Dynamic LST Validation tab, and address any errors or warnings presented in the interface feedback strip at the bottom of the Static Report tab or in the Dynamic LST Validation tab; then you may select the Submission tab (Figure 4.19).

• If the text below the Validate button indicates your project has no errors, then you may click on the Submit button. A pop-up box will appear asking if you are sure you want to submit, click OK.
• Note, the OPT will send an SB submission email to everyone on the project who has selected yes to receive those emails. NRAO staff responsible for checking and validating SBs will receive the same SB submission email.

• If the text below the Validate button indicates your project has errors, then you will need to fix them before the OPT will allow you to submit the SB. The interface feedback strip at the bottom should indicate the errors with the SB to be fixed.
• Note, test projects created by you cannot be submitted. If you try to submit a test project, a pop-up box will appear stating a submission fail and a message in the interface feedback strip at the bottom will give an error message.

Figure 4.19: Example of the Submission Tab

It is important to note here, when the OPT validates an SB it does NOT check the following items:

• allocated science target(s) have been used or have the correct coordinates
• appropriate calibrators have been selected
• correct resource(s) have been selected
• custom resource(s) have been set up correctly, i.e., spectral line

The observer is responsible for correctly setting up their observation(s) and double checking the items listed above. Contact us through the NRAO Science Helpdesk if you have any questions related to SB setup, i.e., need help finding an appropriate complex gain calibrator or advice on the resource setup.

After the SB(s) have been submitted, NRAO staff will check and validate each SB. If we do not find any issues with an SB, it will be approved (changing its status to schedulable — adding it to the dynamic observing queue). Note, we typically do not contact the submitter of valid SB(s). If we do find issues with an SB, we will contact the submitter via the NRAO Science Helpdesk. Please be sure to login to the NRAO Science Helpdesk as least once, so we may find you in the system. To make changes to an SB, you will need to cancel submission (unsubmit).

Cancel Submission

You may cancel submission (unsubmit) an SB at anytime if its status is submitted or schedulable. If the SB status is completed or on hold, you may not cancel submission. (Contact us via the NRAO Science Helpdesk if you need an SB released from the on hold status.)

To cancel submission of an SB:

• select the SB in the left-hand column you wish to unsubmit
• select Submission tab
• select Cancel Submission button
• A pop-up box will appear stating that your SB was successfully cancelled, click Ok.

OPT 1.31 User Interface Update

COSMIC SETI 

Starting in the 23A semester, the COSMIC SETI system will come online at the VLA. 

The Commensal Open Source Multimode Interferometer Cluster (COSMIC) system is a newly developed digital signal processing instrument deployed at the Karl G. Jansky Very Large Array (VLA), which is designed to enable Search for Extra-Terrestrial Intelligence (SETI) observations to be conducted at the VLA in parallel with "Primary User" PI-led Science observations. The COSMIC system capabilities are still under active development.

To facilitate users not wanting SETI to have access to their data, there is an option to opt-out of this data collection in the Submission tab in the OPT. By default this option is selected to be enabled (checked), unchecking this box will opt you out (Figure 4.20). In this case, opting out means that while SETI will still get your raw data, there are no metadata attached to it, e.g., source name, position (RA and Dec), and instrumentation configuration, which renders the received data useless.

Figure 4.20: COSMIC-SETI opt out checkbox (default is to opt in, or checked)